The present invention relates to inkjet inks and methods of reducing bleed in inks employed in inkjet printing. More particularly, the invention relates to an ink system, which includes a pigment-based black ink having cationic and anionic dispersant groups and at least one color dye or pigment that interacts with the black ink when printed on a receiving medium, and use of such inks and dispersant groups to reduce bleed in inkjet inks.
Inkjet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. One such type of printer employs a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern (i.e., a primitive) in a printhead. Each resistor element is associated with a nozzle in a nozzle plate through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an inkjet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Many inkjet inks, when printed in various colors on bond paper, copier paper, and other media, can lead to bleed. Bleed occurs when colors mix on the surface of the paper being printed on and in the paper. As used herein, the term xe2x80x9cbleedxe2x80x9d means that, when inks of two different colors are printed next to each other, one color invades into the other and the border between the two colors becomes ragged. This definition is in contradistinction to uses of the term in the prior art, which often defines xe2x80x9cbleedxe2x80x9d in the context of ink of a single color following the fibers of the paper. Bleed control means that the border between the two colors is substantially clean and free from the invasion of one color into the other.
Prior solutions to bleed have largely involved the use of heated platens or other heat sources and/or special paper. Use of heated platens, however, make the manufacture of printers more expensive. Likewise, use of specialized paper or media, which is more expensive than xe2x80x9cplainxe2x80x9d paper, increases the cost of using the printer. Another way to reduce bleed involves increasing the penetration rate of the ink into the paper. However, increasing the penetration rate reduces the edge acuity (lowers the print quality of the ink). Nevertheless, this method is acceptable for printing color ink because of the lesser importance of color text quality. However, print quality is important for black ink and, hence, alternate bleed control mechanisms are needed.
U.S. Pat. No. 5,428,383 teaches a method to control bleed in multicolor inkjet printing that involves the use of multivalent metal salts as precipitation agents in a first ink composition. The precipitation agent is designed to react with the coloring agent in a second ink composition. A solution to the specific problem of black to yellow color bleed is disclosed in U.S. Pat. No. 5,198,023, wherein multivalent cations such as calcium chloride and magnesium chloride are added at concentrations ranging from about 1 to about 10 wt % to yellow cationic inks to prevent bleed between yellow and black inks. However, it has been determined that the addition of large amounts multivalent cations to thermal inkjet ink compositions may induce precipitation of dye salts, requiring further adjustments in the ink composition.
U.S. Pat. No. 5,518,534 teaches the use of a first ink having a pigment dispersion and a second ink containing a salt of an organic acid or mineral acid wherein said salt reacts with said first ink to alleviate bleed between the inks. However, in this case, the pigment requires the presence of a dispersing agent and both inks must have the same ionic character. U.S. Pat. No. 5,730,790 discloses an inkjet ink that includes at least one dye-based ink composition and at least one pigment-based ink. The dye-base ink also contains a cationic surfactant and the pigment-base ink contains a negatively-charged dispersant.
Another method of reducing bleed between inkjet inks involves the use of pH-sensitive dyes. For example, U.S. Pat. No. 5,181,045 discloses an ink having a pH-sensitive dye to prevent bleeding to an adjacent ink having an appropriate pH. Migration of the ink having the pH-sensitive dye is prevented by rendering the dye insoluble on the page by contact with the pH of the adjacent ink. This method requires a pH differential of about 4 or 5 units to completely control bleed. Accordingly, a pH not exceeding 4 would be required to effectively eliminate bleed from a pH-sensitive ink having a typical pH of about 8. U.S. Pat. No. 5,785,743, builds upon the previously referenced ""045 patent, except that an organic acid component is added to the so-called target inkjet ink composition as opposed to the pH-sensitive ink composition. The organic acid component reduces the pH differential required to control bleed to about 3 units or less.
Each of the above has produced varying degrees of success. However, one common feature of these ink systems is that, as bleed arrest is achieved via the xe2x80x9creactionxe2x80x9d of one ink with the other, if these inks are resident on the same print head, and if the inks inadvertently come in contact with each other on the same print head or on the nozzle plate, they will react and create highly undesirable reliability problems. Reactive inks are likely to form undesirable residues on the nozzle wipers and in spittoons, leading to reliability issues. Therefore, a need remains for ink compositions that are mutually compatible on the print head, while at the same time that are bleed arrested when printed on to the print medium.
The present invention is directed to ink sets that are particularly suitable for inkjet printing over a large range of uncoated papers, have reduced bleed between color and black inks, and include black and color inks that are mutually compatible in pens. The ink sets include at least one pigment-based black ink having cationic and anionic dispersant groups, and at least one color dye or pigment that interacts with the black ink when printed on a receiving medium. The black pigment is self-dispersed and is dual treated with cationic and anionic dispersant groups.
In one particular embodiment of the invention, the black ink includes a pigment that is self dispersed with COOH groups and quaternary amine groups, with the number of COOH groups being greater than the number of quaternary amine groups. The black ink is maintained acidic, the CO2H groups are unionized, and the black ink carries a positive charge (cationic). The color ink component of the ink set may include at least one dye containing at least one quaternary amine group and/or pigment that is dispersed with quaternary amine functional group(s). The color ink is maintained acidic and carries a positive charge (cationic). Due to their similar charges, the black and color inks do not interact in the print head or on the orifice plate. When printed onto an alkaline medium, as the CO2H groups ionize, the black pigment will carry a net negative charge that will interact with the positively charged color dyes and/or pigments and, thus, be bleed arrested.
In another embodiment of the invention, the black ink includes a self-dispersing black pigment and dispersing agents as described with reference to the previous embodiment. In this embodiment, however, the black ink is maintained alkaline and carries a net negative charge (anionic). The color ink component of the ink set may include at least one dye containing at least one SO3H group and/or pigment that is dispersed with SO3H functional group(s). The color ink is maintained alkaline and carries a negative (anionic) charge. Due to their similar charges, the black and color inks do not interact in the print head or on the orifice plate. When printed onto an acidic medium, the black pigment will have a positive charge, will interact with the negatively charged color dyes and/or pigments, and thus be bleed arrested.
The inks of this invention comprise, by weight, from about 0.5 to 20 wt % dye or pigment (total), up to about 50% organic solvent, and the balance mostly water. A buffer may also be employed to maintain the ink at a pH between about 4 and about 9.5 when so desired. Other adjuncts commonly found in inkjet inks, such as a biocides, metal chelating agents may also be added to the inks.